Metal oxide semiconductor field effect transistors (MOSFETs) are electronic devices used in microprocessors and other fast data switching components. Recently, silicon-on-insulator (SOI) device designs have been of particular interest due to the high potential of silicon for high-performance structure fabrication. As the silicon film thickness of SOI transistors is reduced, for high-performance MOS fabrication, many designs increase the thickness of the source/drain regions of the MOSFET device above the initial silicon thickness. This results from the fact that, as the SOI film becomes thinner, there exists a reduced amount of silicon material from which to form silicide for source/drain contacts. Additionally, thinner source/drain regions can degrade on-current due to increased series resistance. The performance of MOSFET devices is limited by the series resistance of the source and drain. The series resistance consists of the spreading resistances in the silicon source/drain and or source/drain extension regions and the contact resistance between the silicon in the source/drain regions and the silicide.
One arrangement that attempts to overcome this concern provides epitaxial growth of silicon on top of the source and drain regions in order to provide additional silicon for silicide. However, a key limitation of this method is that the epitaxial growth has taken place at temperatures as high as 750° C., which causes significant transient enhanced diffusion (TED) of major dopants. The unnecessary dopant redistribution in HALO-extension regions degrades performance of devices and short channel immunity.
Other solutions to reducing the series resistance of the source and drain are to increase the dopant concentration of the source and drain, to reduce the proximity of the silicide to the edges of the channel, or to increase the thickness of the silicide. None of these methods significantly increases the surface area of the interface between the silicide and the silicon, needed to reduce contact resistance. Further, increasing the dopant concentration or reducing silicide proximity degrades short channel effect or enhances junction leakage.